1. Field of the Invention
The present invention relates to a hand-held power tool and, in particular, a hammer drill, a combination hammer and chisel hammer with an active oscillation damper.
2. Description of the Prior Art
An oscillation damper represents an oscillation-capable sub-system that consists of an abstract oscillating mass, an abstract spring, and an abstract damper and that is not necessarily needed to be explicitly formed of concrete components. In particular, the abstract damper often is not formed by concrete components but nevertheless functions by using friction and flow losses which practically always present. There exist two types of dampers, conventional passive oscillation dampers that exclusively are self-excited, and actively controlled dampers which are purposefully independently excited, using primarily a periodic excitation function.
By a suitable selection of spring constant and mass, in a passive damper, its natural frequency can be so dimensioned that it is closed to a to-be-damped interference frequency, in the present case, to the oscillations of the outer housing of the hand-held power tool.
With an active oscillation damper, which is excited primarily with a frequency above the natural frequency by the interference frequency, a good reduction of vibrations of the outer housing is achieved without a damaging beat effect.
French Publication FR 2,237,734 discloses use of a passive oscillation damper for preventing oscillation of a housing of a percussion hand-held power tool.
U.S. Pat. No. 4,478,293 discloses synchronization of operation of two passive oscillation dampers by using compressed air pulsation.
German Publication DE 815 179 discloses an arrangement of two passive oscillation dampers on opposite lateral sides of a percussion mechanism.
European Publication EP 1 710 052 discloses an arrangement of a hollow cylindrical passive oscillation damper around of a guide tube of a percussion mechanism.
International Publication WO 2006/022345 discloses an arrangement of a plurality of compact oscillation dampers within the outer housing of a power tool.
European Publication EP 1 464 449 discloses an arrangement of an actively controlled oscillation damper about a guide tube of a percussion mechanism and which is controlled by pressure fluctuations produced during driving of a drive piston of the power tool. At a faulty pressure of the power tool against an object, in addition to deactivation of the percussion mechanism, the oscillation damper, which is actively controlled by a displaceable control sleeve, is also deactivated.
An object of the present invention is to provide a hand-held power tool having a constructively simple oscillation damper that becomes deactivated at a faulty pressure of the power tool against an object.